Well treatment process

ABSTRACT

The invention provides a method for the treatment of a hydrocarbon well which method comprises administering, down said well, polymeric particles having covalently bound to a polymeric component thereof a well treatment chemical or a precursor thereof, wherein the particles contain covalent bonds scissile in an aqueous environment to release or expose said chemical or precursor.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a 371 of PCT/GB01/05081, filed Nov. 19, 2001, thedisclosure of which is incorporated herein by reference.

This invention relates to a method of treating a hydrocarbon well withwell treatment chemicals, in particular by down-hole placement ofpolymeric particles carrying well treatment chemicals or precursors orgenerators thereof, and to such particles and compositions andstructures containing them.

During the operation of a hydrocarbon well (i.e. a gas or oil well)various down-hole problems arise such as corrosion of metal fittings,hydrocarbon flow-inhibiting deposition (e.g. of scale, gas clathrates,metal sulphides, waxes, gel polymers, microbial debris, etc.),generation of toxic hydrogen sulphide by sulphate-reducing bacteria,increased water flow into the producer bore, etc.

Thus, for example, where sea water is injected through an injection borehole into an oil-bearing stratum to drive oil through the formation(i.e. the rock) into the producer well hole, differences in solutes inthe injection water and the water already present in the formation cancause metal salts to precipitate as scale so causing graduallyincreasing clogging of the producer well hole.

Typically this is dealt with by applying a “squeeze” of scale inhibitorchemicals, i.e. chemicals which break down the scale and increase oil orgas flow. This generally involves ceasing hydrocarbon flow, forcing anaqueous solution of the scale inhibitor down the producer bore underpressure to drive the inhibitor solution into the formation, andrestarting production. Such treatment generally allows a further six orso months of hydrocarbon flow before a further squeeze is required andeach squeeze causes some damage to the formation surrounding theproducer bore hole and as a result an increased flow of formationfragments (i.e. rock grains etc.) into the bore.

The producer bore hole in an oil well is generally lined in thehydrocarbon bearing stratum with “gravel packs”, sand containing filterelements, which serve to trap formation fragments and it has beenproposed to include in such gravel packs ceramic particles coated withor impregnated with well treatment chemicals such as scale inhibitors(see EP-A-656459 and WO 96/27070) or bacteria (see WO 99/36667).Likewise treatment of the formation surrounding the producer well borehole with well treatment; chemicals before hydrocarbon production beginshas also been proposed, e.g. in GB-A-2290096 and WO 99/54592.

Various polymeric, oligomeric, inorganic and other particulate carriersfor well treatment chemicals are also known, e.g. ion exchange resinparticles (see U.S. Pat. No. 4,787,455), acrylamide polymer particles(see EP-A-193369), gelatin capsules (see U.S. Pat. No. 3,676,363),oligomeric matrices and capsules (see U.S. Pat. No. 4,986,353 and U.S.Pat. No. 4,986,354), ceramic particles (see WO 99/54592, WO 96/27070 andEP-A-656459), and particles of the well treatment chemical itself (seeWO 97/45625).

Particles coated or impregnated with or encapsulating a well treatmentchemical however have the inherent problem that release of the welltreatment chemical will take place relatively rapidly once the particlesencounter water down-hole. Accordingly the protection they provide isrelatively short lived.

There is thus a continuing need for well treatments which provideprolonged protection.

Viewed from one aspect the present invention thus provides a method forthe treatment of a hydrocarbon well which method comprises administeringdown said well polymeric particles having covalently bound to apolymeric component thereof a well treatment chemical or a precursorthereof, said particles containing covalent bonds scissile in an aqueousenvironment to release or expose said chemical or precursor.

Viewed from a further aspect the invention provides polymeric particleshaving covalently bound to a polymeric component thereof a welltreatment chemical or a precursor thereof, said particles containingcovalent bonds scissile in an aqueous environment to release or exposesaid chemical or precursor.

Viewed from another aspect the invention provides the use for themanufacture of hydrocarbon well treatment compositions of polymericparticles having covalently bound to a polymeric component thereof awell treatment chemical or a precursor thereof, said particlescontaining covalent bonds scissile in an aqueous environment to releaseor expose said chemical or precursor.

Viewed from a still further aspect the invention comprises a hydrocarbonwell treatment composition comprising a carrier liquid containingpolymeric particles having covalently bound to a polymeric componentthereof a well treatment chemical or a precursor thereof, said particlescontaining covalent bonds scissile in an aqueous environment to releaseor expose said chemical or precursor.

Viewed from a yet further aspect the invention comprises a tubularfilter for down-hole placement containing polymeric particles havingcovalently bound to a polymeric component thereof a well treatmentchemical or a precursor thereof, said particles containing covalentbonds scissile in an aqueous environment to release or expose saidchemical or precursor.

In the method of the invention the polymer particles may be placed downhole before and/or after hydrocarbon production (i.e. extraction of oilor gas from the well) has begun. Preferably the particles are placeddown hole before production has begun, especially in the completionphase of well construction.

The particles may be placed within the bore hole (e.g. in thehydrocarbon bearing strata or in ratholes) or within the surroundingformation (e.g. in fissures or within the rock itself). In the formercase, the particles are conveniently contained within a tubular filter,e.g. a gravel pack or a filter structure as disclosed in EP-A-656459 orWO 96/27070; in the latter case, the particles are preferably positionedby squeezing a liquid composition containing the particles down the borehole. Preferably, before production begins the particles are placed bothwithin the bore in a filter and within the surrounding formation.

Where the particles are placed within the surrounding formation, thepressure used should be sufficient to cause the particles to penetrateat least 1 m, more preferably at least 1.5 m, still more preferably atleast 2 m, into the formation. If desired, the particles may be appliedin conjunction with proppant particles (e.g. as described in WO99/54592) to achieve a penetration of up to about 100 m into theformation. Compositions comprising proppant particles and polymerparticles according to the invention form a further aspect of theinvention.

The polymer particles of the invention, which may be porous ornon-porous but preferably are porous, may have a wide range of possiblestructures, several of which are illustrated schematically in theaccompanying drawings. Thus for example,

FIG. 1 illustrates the case where the chemical or precursor is bondedvia a scissile bond to the polymer matrix;

FIG. 2 illustrates the case where the chemical or precusor is itself acomponent of a polymeric or oligomeric part of the particle which isdegradable to release the chemical or precursor;

FIG. 3 illustrates the case where the chemical or precursor is areleasable part of the overall polymer matrix; and

FIGS. 4 and 5 illustrate cases where the scissile bonds are present inthe backbone or cross-links of a polymer matrix to which the chemical orprecursor is covalently bound, whereby bond scission breaks down theparticle exposing the chemical or precursor. In these schematicillustrations, dotted lines represent scissile bonds, continuous linespolymer or oligomer backbones and X the chemical or precursor.

The scissile bonds in the polymer particles of the invention may be anybonds subject to scission in the presence of formation or injectionwater under the temperature conditions experienced down hole, e.g. 70 to150° C. Suitable such bonds include amide, ester, disulphide, diester,peroxide, etc. bonds. These bonds may be formed by conjugation of thechemical or precursor to a functionalized polymer matrix (e.g. onehaving unsaturated carbon-carbon bond, or pendant hydroxy, thiol, amineor acid groups), or by the oligomerization or polymerization process, orthey may be incorporated within a monomer or comonomer used in theoligomerization or polymerisation process, they may be incorporatedwithin a cross-linking agent or formed by a cross-linking reaction, orthey may be incorporated in the reagent which serves to introduce thechemical or precursor, i.e. a compound A-B-C where A is a moiety whichbinds to the polymer matrix, B is the scissile bond and C is thechemical or precursor moiety.

If desired, the particles may be impregnated with agents which, underdown-hole conditions, will promote scission of the scissile bonds, e.g.enzymes, acids, bases, metal complexes, etc.

The rate of chemical or precursor release can accordingly be selected orcontrolled by selection of the properties of the particles (e.g. choiceof monomer, degree of cross-linking, polymer molecular weight, particlesize, porosity, nature of the scissile bonds, nature of other materialsimpregnated into the particles, etc.) to match properties of thedown-hole environment, e.g. temperature, salinity, pH etc.

The particles according to the invention advantageously have modeparticle sizes (e.g. as measured with a Coulter particle size analyser)of 1 μm to 5 mm, more preferably 10 μm to 1000 μm, especially 250 to 800μm. For placement within the formation, the mode particle size ispreferably 1 to 50 μm, especially 2 to 20 μm. For any particularformation, formation permeability (which correlates to the pore throatsizes in the formation) may redily be determined using rock samplestaken during drilling and the optimum particle size may thus bedetermined. If the particles have a low dispersity (i.e. sizevariation), a highly uniform deposition and deep penetration into theformation can be achieved. For this reason, the particles preferablyhave a coefficient of variation (CV) of less than 10%, more preferablyless than 5%, still more preferably less than 2%.

CV is determined in percentage as

${CV} = \frac{100 \times {standard}\mspace{14mu}{deviation}}{mean}$where mean is the mean particle diameter and standard deviation is thestandard deviation in particle size. CV is preferably calculated on themain mode, i.e. by fitting a monomodal distribution curve to thedetected particle size distribution. Thus some particles below or abovemode size may be discounted in the calculation which may for example bebased on about 90% of total particle number (of detectable particlesthat is). Such a determination of CV is performable on a Coulter LS 130particle size analyzer.

For placement in filters, the particles preferably have mode particlesizes of 50 to 5000 μm, more especially 50 to 1000 μm, still morepreferably 100 to 500 μm. In such filters, the particles preferablyconstitute 1 to 99% wt, more preferably 2 to 30% wt, still morepreferably 5 to 20% wt of the particulate filter matrix, the remainingmatrix comprising particulate oil- and water-insoluble inorganicmaterial, preferably an inorganic oxide such as silica, alumina oralumina-silica. Particularly preferably, the inorganic oxide has a modeparticle size which is similar to that of the polymer particles, e.g.within 20%, more preferably within 10%. As with the in-formationplacement, the polymer particles preferably have low dispersity, e.g. aCV of less than 10%, more preferably less than 5%, still more preferablyless than 2%. The low dispersity serves to hinder clogging of thefilters.

Preferably the polymer matrix of the particles has a softening pointabove the temperatures encountered down hole, e.g. one above 70° C.,more preferably above 100° C., still more preferably above 150° C.

The well treatment chemicals or precursors thereof which the particlescontain may be any agents capable of tackling down hole problems, suchas corrosion, hydrocarbon flow reduction, or H₂S generation. Examples ofsuch agents include scale inhibitors, foamers, corrosion inhibitors,biocides, surfactants, oxygen scavengers, etc.

The particles may contain a well treatment chemical itself or aprecursor chemical compound which in situ will react, e.g. break down,to produce a well treatment chemical, or alternatively it may be abiological agent, e.g. an enzyme which produces a well treatmentchemical.

Examples of typical well treatment chemicals, precursors and generatorsare mentioned in the patent publications mentioned herein, the contentsof all of which are hereby incorporated by reference.

Thus for example typical scale inhibitors include inorganic and organicphosphonates (e.g. sodium aminotrismethylenephosphonate),polyaminocarboxylic acids, polyacrylamines, polycarboxylic acids,polysulphonic acids, phosphate esters, inorganic phosphates, polyacrylicacids, inulins (e.g. sodium carboxymethyl inulin), phytic acid andderivatives (especially carboxylic derivatives) thereof, polyaspartates,etc.

Examples of preferred well treatment chemicals include: hydrateinhibitors, scale inhibitors, asphaltene inhibitors, wax inhibitors andcorrosion inhibitors. Such inhibitors are well known to those working inthe field of well treatment.

Where the partices are placed within the formation, they are preferablyapplied as a dispersion in a liquid carrier. For pre- andpost-completion application, the liquid carrier preferably comprises anon-aqueous organic liquid, e.g. a hydrocarbon or hydrocarbon mixture,typically a C₃ to C₁₅ hydrocarbon, or oil, e.g. crude oil. For curativetreatment, i.e. after production has continued for some time, the liquidcarrier may be aqueous or non-aqueous.

The invention will now be described further with reference to thefollowing non-limiting Examples:

EXAMPLE 1

1.8 g of methacrylic acid anhydride, 4.2 g of diethylvinyl phosphonate,and 4 g of toluene are mixed and 0.3 g of dibenzoylperoxide is added tothe mixture. This oil phase is dispersed in a solution of 0.03 g of87-89% hydrolysis grade polyvinylalcohol in 70 g water in a reactor. Theresulting suspension is stirred at 150 rpm at 80° C. for 6 hours afterwhich the resulting suspension polymerized particles are removed byfiltration, washed with toluene and dried.

1. A method for the treatment of a hydrocarbon well which methodcomprises administering, down said well, polymeric particles havingcovalently bound to a polymeric component thereof a well treatmentchemical or a precursor thereof, said particles containing covalentbonds scissile in an aqueous environment to release or expose saidchemical or precursor.
 2. The method as claimed in claim 1, wherein saidpolymer particles are administered before hydrocarbon production fromsaid well begins.
 3. The method as claimed in claims 1 or 2, whereinsaid polymer particles are placed in a filter in the bore-hole of saidwell and in the formation surrounding said bore-hole.
 4. The method asclaimed in claim 1, wherein said polymer particles are impregnated withan agent capable of promoting scission of said scissile covalent bonds.5. A tubular filter for down-hole placement containing polymericparticles having covalently bound to a polymeric component thereof awell treatment chemical or a precursor thereof, wherein said particlescontain covalent bonds scissile in an aqueous environment to release orexpose said chemical or precursor.
 6. The filter as claimed in claim 5,wherein said polymer particles are impregnated with an agent capable ofpromoting scission of said scissile covalent bonds.